1. Field of the Invention
The present invention relates to a color toner for use in developing electrostatic images formed by a method such as electrophotography, electrostatic recording and electroprinting. In addition, the present invention also relates to a toner container containing the toner, a process cartridge using the toner, and an image forming apparatus and an image forming method using the toner.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes:    (1) an electrostatic latent image is formed on an image bearing member (electrostatic latent image forming process);    (2) the electrostatic latent image is developed with a charged toner to form a visual toner image on the image bearing member (developing process);    (3) the toner image is transferred onto a receiving material optionally via an intermediate transfer medium (transfer process); and    (4) the toner image on the receiving material is fixed upon application of heat, pressure and/or a solvent thereto, resulting in outputting of a hard copy (fixing process).
Recently, not only electrophotographic monochrome copiers and printers but also electrophotographic full color copiers and printers are commercialized. It is considered that the market of the electrophotographic full color copiers and printers will become active more and more.
Color images are typically formed by overlaying three primary color toner images (i.e., a yellow toner image, a magenta toner image and a cyan toner image) and a black toner image. In order to form a color image having good color reproducibility and clearness, the fixed color toner image preferably has a smooth surface to some extent to avoid excessive light scattering on the color image. Therefore, color images produced by conventional full color copiers typically have a medium gloss of from 10% to 50%.
In general, contact heating fixing methods in which a dry toner image is fixed on a receiving material upon application of heat thereto using a heat fixing member having a smooth surface, such as heated rollers and belts, are typically used as the toner image fixing method. These methods have advantages such that the heat efficiency is high and high speed fixing can be performed, thereby imparting high gloss and transparency to the toner image. However, since the melted toner is brought into contact with the heat fixing member upon application of pressure thereto and then the toner is peeled from the heat fixing member, the methods tend to cause an offset problem in that a part of the toner image on a receiving material is transferred to the surface of the heating member and is then re-transferred onto other images, resulting in deterioration of image qualities.
In attempting to avoid such an offset problem, fixing methods in which a layer of a material having good releasability such as silicone rubbers and fluorine containing resins is formed on the surface of the heat fixing member and in addition a release agent such as silicone oils is applied to the surface of the heat fixing member have been used. These fixing methods are effective at preventing occurrence of the offset problem, but a device applying a release agent has to be provided, resulting in increase in size of the fixing device and increase of manufacturing costs of the fixing device.
Recently, fixing methods in which no release oil or a very small amount of a release oil is applied to a heat fixing member have been used while using a monochrome toner which has a high melt viscoelasticity by controlling the molecular weight distribution of the binder resin to prevent internal breaking of the toner and which includes a release agent such as waxes.
However, as mentioned above, color toners have to have a low melt viscoelasticity to smooth the surface of the resultant color toner images (i.e., to impart good color reproducibility to the resultant color toner images). Therefore, color toners tend to cause the offset problem compared to monochrome toners because the monochrome toner images having a low gloss do not cause any problem. Therefore, it is hard to use an oil-free fixing device or a fixing device in which a small amount of oil is applied to a heat fixing member for color copiers using color toners.
In addition, when a release agent is added to a toner, the following problems tend to occur    (1) the toner has a large adhesion on the image bearing member and therefore the transfer ratio of toner images transferred from the image bearing member to a receiving material decreases; and    (2) the release agent included in the toner contaminates the frictional-charging members such as carriers of the toner, resulting in deterioration of charging properties of the charging members, i.e., deterioration of durability of the charging members.
Conventionally, resins such as polyester resins and epoxy resins have been used as binder resins for color toners because the resultant toners can produce images having a high gloss even when the resins have a low molecular weight. However, such resins have a drawback such that the charges of the resultant toners largely change when environmental humidity changes. This is because the resins have a hydrophilic group therein. In addition, it is a recent trend that toner particles are miniaturized to produce high quality images. In this regard, polyester resins and epoxy resins have poorer pulverizability than styrene resins which have been typically used as binder resins for monochrome toners.
In attempting to provide a toner which can exhibit a good combination of pulverizability, transferability, durability and charge stability and which does not cause an offset problem even when used in fixing methods in which no release oil or a small amount of release oil is applied to a fixing member, the following toners have been proposed:    (1) a toner including a linear polyester resin having a softening point of from90 to 120° C. and a carnauba wax (Published Unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 08-220808);    (2) a toner including a resin and a wax which are mixed with each other and which have different softening points (JP-A 09-106105);    (3) a toner including a polyester resin having a specific melt viscosity and a wax having a specific melt viscosity (JP-A 09-304964);    (4) a toner including a polyester resin having a softening point of from 90 to 120° C., a rice wax, a carnauba wax and a silicone oil (JP-A 10-293425); and    (5) a polymerized toner in which a wax is included in a resin particle (JP-A 05-61242).
However, these toners are not a toner which can exhibit a good combination of pulverizability, transferability, durability and charge stability and which does not cause an offset problem even when used in fixing methods in which no release oil or a small amount of release oil is applied to a fixing member.
Recently, the fixing temperature of fixing devices is decreased to save energy, namely, the energy applied to a toner image during the fixing process decreases. Therefore, it is essential for a toner to have a low temperature fixability so as to be used for low temperature fixing devices. In order to provide a toner having a low fixable temperature, the thermal properties of the binder resin used in the toner has to be controlled. However, when a resin having too low a glass transition temperature (Tg) is used as a binder resin, the high temperature preservability of the resultant toner deteriorates. When the F1/2 temperature of a resin, which is defined as the mid-temperature of the flow-starting temperature and the flow-ending temperature, is excessively decreased by decreasing the molecular weight of the resin, problems such that the lower limit of the hot offset temperature range decreases and in addition the gloss of the resultant images seriously increases occur. Namely, a toner which has a good low temperature fixability and a high hot offset temperature and which can produce images having a proper gloss cannot be provided by merely controlling the thermal properties of the binder resin used.
In attempting to improve the low temperature fixability, JP-As 60-90344, 64-15755, 02-82267, 03-229264, 03-41470 and 11-305486 have disclosed techniques such that a polyester resin having a good low temperature fixability and a fair high temperature preservability is used as a binder resin. Further in attempting to improve the low temperature fixability, JP-A 62-63940 discloses a technique such that a specific non-olefin crystalline polymer which sharply melts at its glass transition temperature is included in the binder resin. However, the toners prepared by these techniques are not a toner which can exhibit a good combination of pulverizability, transferability, durability and charge stability and which does not cause an offset problem even when used in fixing methods in which no release oil or a small amount of release oil is applied to a fixing member.
Japanese patent No. 2,931,899 and JP-A 2001-222138 have disclosed techniques such that a crystalline polyester resin having a sharp-melting property is used as the binder resin. In the technique disclosed in JP 2,931,899, the acid value and hydroxyl value are as low as not greater than 5 and not greater than 20, respectively. Therefore the polyester resin has poor affinity for receiving papers, and thereby a good low temperature fixability cannot be imparted to the resultant toner. In addition, the molecular structure and molecular weight of the crystalline polyester resins are not optimized. Therefore, a toner which has a good low temperature fixability and good offset preventing property even when used for fixing devices in which no release oil or a very small amount of a release oil is applied to a fixing element and which has good transferability, good durability, good charge stability (i.e., little charge dependence on humidity), and good pulverizability is not provided.
By using the technique disclosed in JP-A 2001-222138, a toner which has a good low temperature fixability and good offset preventing property even when used for fixing devices in which no release oil or a very small amount of release oil is applied to a fixing element and which has good transferability, good durability, good charge stability (i.e., the dependence of the charge quantity of the toner on humidity is little), and good pulverizability cannot be provided.
Because of these reasons, a need exists for such a toner as having a good combination of the above-mentioned properties.